Pilot-operated diaphragm valves are customarily used as zone valves in irrigation systems and are broadly characterized by having a pilot control which cooperates with the diaphragm to regulate opening and closing of the valve so that if there is line pressure through the inlet the diaphragm is held in a closed position by permitting liquid to fill the diaphragm chamber behind the diaphragm; and, when the solenoid is activated to open the valve, the liquid is drained out of the diaphragm chamber until the pressure in the inlet line exceeds that of the diaphragm and will advance the diaphragm away from its closed position. In the past, when valves of this type have required servicing, it has been necessary to turn off line pressure to all of the valves so that the irrigation operation is totally interrupted until the defective valve can be serviced or repaired and placed back into operation. Not only is this time-consuming but can cause lengthy interruptions in the irrigation cycle and loss of any or all water present in the mainline. Also any dirt or debris washed about by draining the mainline water can be trapped in the valve body causing more problems when the system is recharged.
Representative systems and valves which have been devised in the past are disclosed in various U.S. patents including U.S. Pat. No. 3,967,808 to C. A. Lieding in which a valve can either be manually opened or electrically controlled by a solenoid to open and close but does not independently close either one of its inlets to permit maintenance or servicing of the valve. U.S. Pat. Nos. 3,367,621 and 3,556,464 to D. E. Griswold disclose diaphragm valves which can be modulated to operate between partially open positions but does not permit movement to a fully closed position for servicing of the valve independently of other parts of the system.
It is therefore desirable to be able to selectively close a valve against line pressure to permit servicing of the valve without interrupting the supply of liquid to other parts of the system or to other systems which are operated off of the same water source. Thus, it is important to provide in valves of the type described for a highly simplified means of regulating flow through the valve without affecting the rest of the system so that water flow can be individually controlled or regulated according to the needs of each area being irrigated.